Improved optical cavity quality of a waveguide free-electron laser with hole coupling by using a grating

A method is proposed to improve the quality of a hole- coupled resonator for a free-electron laser (FEL) oscillator. By using a mesh or grating with a moderate reflectivity to cover the hole, the optical cavity Q can be effectively improved, as is confirmed by three-dimension numerical simulations. The potentiality of the mesh as an electron- transparent mirror to make the FEL device more compact and less expensive is discussed.